1. Field of the Invention
The invention relates generally to the field of digital signal communications and to receive and transmit chain calibration. More particularly, the invention relates to calibrating a wireless base station using statistical operations on an accumulated set of received signals.
2. Description of the Related Art
Radio communications capacity can be greatly increased using directional, rather than omnidirectional radio transmission. One way to transmit directional signals and directionally receive signals is by using beam forming and nulling through an array of antennas. The precision of the beam forming and nulling using an antenna array, can be improved if the transmit and receive chains, from the digital interface at baseband to the field radiated from each antenna element is calibrated. One way of making the calibration is to have a remote, portable RF transponder listen to the output of the antenna array on a base station downlink frequency and re-transmit the downlink signal that it receives from the base station, on the base station's uplink frequency. By selecting appropriate signals to transmit and appropriate signals to receive, the base station can apply signal processing to estimate compensations in phase and amplitude to calibrate its transmit and receive chains.
Calibration signal processing usually includes estimating the phase and amplitude response of the receive section of each antenna from the received electric field at the antenna, to the input to the A/D (analog to digital) converter of the baseband circuitry. The phase/amplitude pair for each antenna receive section forms a complex number, and the collection of these for all the antennas forms a complex valued receive-section calibration vector. Similarly, the phase and amplitude response of the transmit section of each antenna (from the output of the baseband transmit circuitry to the transmitted electric field at the antenna) can be grouped into a complex-valued transmit-section calibration vector.
These calibration vectors tend to change slowly over time due to environmental changes such as temperature, and also over frequency bands, but can change so much that unless they are taken into account, signals transmitted from an antenna array will emanate from each antenna with unknown phase, and therefore reduce the effectiveness of directionally transmitting coherent gain toward desired remote user terminals or nulls to interfering remote user terminals. Since the phase and amplitude response of both the receive and transmit sections depends upon installation-specific items such as cables, connectors, antennas, etc., it is usually not possible to accurately calibrate a system in the factory. For maximum accuracy, calibration is performed “on site”, at a rate that tracks the changes.
A remote transponder calibration system is shown, for example, in U.S. Pat. No. 5,546,090 to Roy, III et al. That patent describes a narrowband FDD (frequency division duplex) system that uses a remote transponder for calibration. Such a technique requires external equipment including additional radios and either directional couplers or additional antennas mounted somewhere visible to the base station. This adds the cost of the additional equipment as well as the difficulty in locating the calibration antenna.
For TDD (time division duplex) systems, a similar calibration process can be used, i.e. a transponder can be used to echo back calibration signals. However, with some antenna configurations, it is possible to reuse the same base station hardware that is normally used for regular traffic. Such an approach is shown, for example, in U.S. Pat. No. 6,037,898 to Parish et al. However this method may not work with some directional antenna systems, such as sectorized antenna systems. Sectorized antenna systems are commonly constructed, for example, as a linear array.